Notes from the linux-sig thread at: https://mail.python.org/pipermail/linux-sig/2017-January/000014.html

• change the sense of the environment variable to be an explicit off-switch for the coercion behaviour (PYTHONCOERCECLOCALE=0) rather than being an on-switch for more permissive behaviour
• add a "--with[out]-c-locale-coercion" configure flag and a related PY_COERCE_C_LOCALE preprocessor definition (for the command line app coercion behaviour)
• add a "--with[out]-c-locale-warning" configure flag and a related PY_WARN_ON_C_LOCALE preprocessor definition (for the runtime library warning behaviour)
• in the command line app, check the result of the C.UTF-8 locale coercion, complain if it doesn't work, and then try the en_US.UTF-8 locale. If that still doesn't work, complain again, then continue with the C locale
• simplify the library warning to:

Python runtime initialized with LC_CTYPE=C (a locale with default ASCII encoding), which may cause Unicode compatibility problems. Using C.UTF-8 as an alternative Unicode-compatible locale is recommended.